The U.S. stationary power industry is continually being required to lower its pollution levels, especially those of nitrogen oxide gas (NOx), to very low levels. Currently, the industry standard for NOx emissions ranges from 3 to 20 parts per million (ppm). However, federal and state environmental protection agencies are now demanding exhaust emission levels to 3 ppm or below in most areas of the country. Thus, a combustor delivering NOx levels below 1 ppm would likely become certified as the “Best Available Technology” for lowering NOx levels.
Some prior art combustors employ a single central diffusion pilot swirler operating at equivalence ratios between 0.8 and 1.2 surrounded by eight premix swirlers operating at equivalence ratios of approximately 0.4. Other prior art combustors employ multiple large scale (>1.0 inch) swirl injectors and pre-mixers to minimize NOx formation. At best, however, such technology generally obtains NOx emissions as low as 25 ppm. Still other previous devices employ air/fuel premixing within the vanes of a swirler while injecting the pilot fuel from the swirler's center nozzle in order to stabilize the swirler's central diffusion flame. Because the fuel/air mixing distances with these swirler's are lower than those of the other previous devices, this approach can achieve NOx emissions down to about 9 ppm, but no further.
One source of NOx emissions occurs in the high temperature recirculation flame holding zones created by the prior art pilot injectors. These pilot injectors produce the bulk of the combustor's NOx in these flame holder areas. Clearly, such technology falls short of the current upper limit of 3 ppm NOx emissions.
In a typical combustor, air from a multistage axial compressor is discharged radially outward and around the annular combustor. The air flows through the swirl injectors and mixes and burns with the natural gas fuel in the combustor. The combustion product gas is subsequently expanded through a multi-element axial expander (i.e. a turbine) and exhausted to atmosphere.
These swirlers are highly susceptible to acoustic combustion instability. This is because gas turbines require very low air side pressure drops across the combustion chamber in order to maintain high turbine operating efficiencies. Swirler combustors essentially take all of the air-side pressure drop at the axial location of the swirler. Accordingly, the combustion chamber and air plenums within the combustor lack sufficient damping to avoid acoustical instabilities.
Furthermore, pre-mix swirlers are very susceptible to detrimental combustion flashback during turn-down and start-up operation because of their large characteristic dimension (e.g., above 1.0 inch). For proper combustor operation, all combustion is designed to take place downstream of the swirlers where there is sufficient air film cooling and ceramic coatings to protect the combustor walls. Flashback, via the associated internal combustion in the swirler, frequently damages the swirler by overheating it.
Another problem with the previously implemented swirl combustors occurs during power turn-down wherein the overall fuel flow is reduced to equivalence ratios below 0.4. Industry currently handles turn-down operation by shutting off the fuel to selected pre-mix swirlers in order to maintain stable combustion. However, the air from these non-fueled swirlers rapidly mixes with and cools the combustion products from the active pre-mix swirlers before the partially combusted carbon monoxide (CO) can be effectively oxidized to CO2. Hence, during turndown operation, swirler combustors inherently produce excessive amounts of carbon monoxide (well above the adiabatic equilibrium combustion amounts of approximately 3 ppm).
Thus, a need exists to improve the previous combustors, particularly with regard to NOx and CO emissions. Additionally, a need exists to reduce, or eliminate, the acoustic instabilities and flashback that conventional combustors suffer from. Furthermore, a need exists for the capability to burn essentially pure hydrogen fuel having very high air premix flame speeds to reduce the occurrence of detrimental flashbacks and burnouts known to occur in conventional swirler injectors.